Scientists Identify Genes Key To Differentiating Top From Bottom In Plant Leaves

PHILADELPHIA – Biologists at the University of Pennsylvania and the University of Wisconsin have identified some of the first genes known to have a hand in differentiating top from bottom in plant leaves, a subtle morphological distinction that has profound implications for development and function across a wide range of plant species.

The Penn researchers describe the function of a gene called KANADI in the June 7 issue of the journal Nature. KANADI is expressed primarily on the underside of leaves; in a companion paper, the Wisconsin researchers describe a related gene called PHABULOSA, which is active in cells closer to leaves’ upper surfaces.

Described by the scientists in the plant Arabidopsis thaliana, variations of the two genes are believed to exist in plants from snapdragons to corn.

Anyone who’s ever paid attention to trees being buffeted on a stormy day has likely noticed that the tops of leaves aren’t quite the same as their undersides. Since most photosynthesis takes place in cells on the top side of a leaf, that side is densely packed with cylindrical cells containing chloroplasts, yielding a brighter hue than on a leaf’s grayer bottom. In some leaves, the upper surfaces are also marked by minuscule hairs called trichomes.

While the variations might appear inconsequential to the untrained eye, these differences foster the distinct – and very important – roles the two sides of the leaves play in plants, said R. Scott Poethig, senior author on the Penn Nature paper.

"Most photosynthesis takes place on the upper side of a leaf, since that side spends most of its time oriented toward the sun," said Poethig, a professor of biology in Penn’s Plant Science Institute. "The underside of a leaf, which is less densely packed with cells and has many epidermal pores, is a plant’s main interface for the exchange of gases and water with the environment."

In plants mutant in KANADI, Poethig and his colleagues found that the bottoms and tops of leaves were more or less identical. While the group’s research suggests that KANADI may work in conjunction with other genes, including the one described by Kathy Barton and her colleagues at Wisconsin, it appears to be a key player in establishing dorsal-ventral polarity in plant leaves. Poethig’s paper reports that KANADI helps establish polarity in fruits as well as leaves.

KANADI was first identified by researchers at the University of California, Davis, who gave the gene a name meaning "mirror" because of its effects on the structure of the seed pod. In wild-type individuals, Poethig’s team found the RNA encoded by the gene only in the lower surfaces of leaves and parts of the flower, and in the outermost layers of young embryos.

Poethig was joined in the research by Randall A. Kerstetter, Krista Bollman, R. Alexandra Taylor and Kirsten Bomblies of Penn’s Plant Science Institute. Their work was funded by the National Institutes of Health and the U.S. Department of Energy.